The present invention relates to transistor amplifiers and in particular to a high-power amplifier employing heterojunction bipolar transistors (HBTs) with common-base (CB) configurations.
High power amplification at high frequencies, for example, such as is required for radio frequency communication, often uses heterojunction bipolar transistors (HBTs) in the final power stages of the signal amplification. HBTs differ from bipolar junction transistors (BJTs) in that HBTs use different materials for the emitter and bass regions creating a heterojunction. This improvement generally increases the operating frequency of the transistor up to several hundred gigahertz.
Generally the power handling capability of any transistor depends to a first order on the area of the transistor on the substrate. Increased demand for miniature radio transmitters having decreased power consumption places a premium on minimizing the area required for the transistor devices. Accordingly it is desirable to produce transistor amplifiers having the greatest possible power handling capability for a given transistor area.
Two common power amplifier configurations are the common-base (CB) and common-emitter (CE) configurations. In the common-base configuration, the input signal is applied to the emitter of the transistor and the base is at signal ground. In the common-emitter configuration, the input signal is applied to the base of the transistor and the emitter is at signal ground. In both cases the output signal may be obtained at the collector of the transistor.
For transistors of the same size, it was widely believed that CB and CE amplifiers should provide identical power. In this case, CB transistors may be preferred for some high-frequency circuits because this configuration provides high voltage gain even at high frequencies where current gain quickly drops for the corresponding CE transistors that have smaller voltage gain than CB transistors. See, for example, co-pending U.S. application Ser. No. 11/178,644 entitled: “High-Power-Gain, Bipolar Transistor Amplifier” filed Jul. 11, 2005, assigned to the same assignee as the present invention and hereby incorporated by reference.
Surprisingly, present inventors determined that the power handling capabilities for HBTs are significantly affected by whether they are used in a CB or CE configuration. In particular, in voltage source biasing, CE amplifiers always provide higher (saturated) output power than CB configurations.
This finding is discussed in H. Li, N. Jiang, G. Wang and Z. Ma, Fundamental Difference of Power Handling Between CE and CB HBTs, Digest of Papers of 2006, 28th IEEE Compound Semiconductor IC (CSIC) Symposium, San Antonio, Tex., Nov. 12-15, 2006, pp. 183-186.